The invention relates to electrochemical cells. More particularly, it relates to electrochemical cells which have intermediate performance between high energy and high power cells.
Most of the cells made today with lithium anodes and either inorganic- or organic-based electrolytic solutions have their electrode structures packaged in a wound, spiral form in cylindrical containers. The low conductivity of the organic-based electrolytic solutions dictated such configurations in an effort to increase cell rate capabilities by providing increased surface area and closer packing of the electrodes. Such spiral electrode configurations were maintained during the period in which substantially more conductive electrolytic solutions were developed, the most important of them being formed from sulfur dioxide as the principal electrolytic solvent. The emergence of this sulfur dioxide-based electrolytic material, having a conductivity comparable to that of the best organic electrolytic solutions, but with some additional desirable characteristics, offered many alternatives in the area of cell design and construction.
In copending application Ser. No. 685,214, filed May 11, l976, now abandoned, a continuation of application Serial No. 492,316, filed July 26, 1974, now abandoned, there are described electrochemical cells having an oxidizable active anode material, such as lithium, a carbon cathode, and an electrolytic solution between and in contact with the anode and cathode materials. The electrolytic solution includes an inorganic oxyhalide solvent, such as phosphorus oxychloride, monofluorophosphoryl dichloride, thionyl chloride, sulfuryl chloride, or mixtures thereof, and a solute dissolved therein to render the electrolytic solution ionically conductive. It was found that the carbon cathode material catalyzes the electrochemical decomposition of the solvent during discharge of the cell, thereby enabling the otherwise "dead" weight of the solvent to be utilized as a source of electrical energy.
In application Ser. No. 515,557, filed Oct. 24, 1974, now U.S. Pat. No. 3,923,543, there are described electrochemical cells having an oxidizable active anode material, such as lithium, a cathode, including, as the active cathode material, an intercalation compound of carbon and fluorine of the general formula (C.sub.4 F).sub.n, where n refers to the presence of a large, but indefinite, number of recurring (C.sub.4 F) groups in the intercalation compound, and an electrolytic solution between and in contact with the anode and cathode materials. The electrolytic solution includes an inorganic oxyhalide solvent selected from those set forth above in the preceeding paragraph, and a solute dissolved therein to render the electrolytic solution ionically conductive. It was found that the above-identified intercalation compound catalyzed the electrochemical decomposition of the solvent resulting, unexpectedly, in a cell having a coulombic cathode utilization efficiency greater than 100 percent of that theoretically attainable according to the reduction of the active cathode material. As with the cells described in copending application Ser. No. 685,214, the otherwise "dead" weight of the solvent is utilized as a source of electrical energy.
Cells fabricated from the components described in the above-identified copending applications, particularly those with carbon cathodes and concentrically arranged electrodes, demonstrated exceptionally high energy densities at low discharge rates, reaching 440 Wh/kg in small cells (i.e., size AA) and extending to 550 Wh/kg in a 150 Ah size cell. Cells of this configuration have been applied successfully on a commercial scale in implantable devices, such as cardiac pacemakers.
Cells fabricated from the components described in the above-identified copending applications, particularly those with carbon cathodes and spirally-wound electrodes, demonstrated high power capabilities due to approximately an order of magnitude more electrode surface over the high energy model of the same volume. To date, such high power cells have been used only for military applications.
A study was conducted comparing and evaluating the particular design characteristics and cell capabilities for cells of constant volume but for various discharge rate capabilities desired. It was determined that, for the two basic cell configurations referred to above (i.e., concentrically arranged electrodes and spirally-wound electrodes), there existed a gap in the cell discharge rate capabilities which could be attained with these basic cell configurations. A substantial reduction in the maximum discharge rate capability is predicted when the concentric electrode structure is converted into a spirally-wound structure where the number of turns is between 1 and 2. Thus, if there is a need for a cell having twice the discharge rate capability obtainable with the concentric electrode structure, such a need cannot be fulfilled efficiently with a spirally-wound electrode configuration. It, therefore, would be desirable to provide an electrochemical cell which closes the gap in performance between the high energy cells and the high power cells referred to above.